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<title>Chapter 6. Boost.Atomic</title>
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<div class="chapter">
<div class="titlepage"><div>
<div><h2 class="title">
<a name="atomic"></a>Chapter 6. Boost.Atomic</h2></div>
<div><div class="author"><h3 class="author">
<span class="firstname">Helge</span> <span class="surname">Bahmann</span>
</h3></div></div>
<div><div class="author"><h3 class="author">
<span class="firstname">Andrey</span> <span class="surname">Semashev</span>
</h3></div></div>
<div><p class="copyright">Copyright © 2011 Helge Bahmann</p></div>
<div><p class="copyright">Copyright © 2012 Tim Blechmann</p></div>
<div><p class="copyright">Copyright © 2013, 2017, 2018, 2020, 2021 Andrey Semashev</p></div>
<div><div class="legalnotice">
<a name="atomic.legal"></a><p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
</div></div>
</div></div>
<div class="toc">
<p><b>Table of Contents</b></p>
<dl class="toc">
<dt><span class="section"><a href="atomic.html#atomic.introduction">Introduction</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="atomic.html#atomic.introduction.introduction_presenting">Presenting
      Boost.Atomic</a></span></dt>
<dt><span class="section"><a href="atomic.html#atomic.introduction.introduction_purpose">Purpose</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="atomic/thread_coordination.html">Thread coordination using Boost.Atomic</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="atomic/thread_coordination.html#atomic.thread_coordination.mutex">Enforcing <span class="emphasis"><em>happens-before</em></span>
      through mutual exclusion</a></span></dt>
<dt><span class="section"><a href="atomic/thread_coordination.html#atomic.thread_coordination.release_acquire"><span class="emphasis"><em>happens-before</em></span>
      through <code class="literal">release</code> and <code class="literal">acquire</code></a></span></dt>
<dt><span class="section"><a href="atomic/thread_coordination.html#atomic.thread_coordination.fences">Fences</a></span></dt>
<dt><span class="section"><a href="atomic/thread_coordination.html#atomic.thread_coordination.release_consume"><span class="emphasis"><em>happens-before</em></span>
      through <code class="literal">release</code> and <code class="literal">consume</code></a></span></dt>
<dt><span class="section"><a href="atomic/thread_coordination.html#atomic.thread_coordination.seq_cst">Sequential consistency</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="atomic/interface.html">Programming interfaces</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="atomic/interface.html#atomic.interface.configuration">Configuration and building</a></span></dt>
<dt><span class="section"><a href="atomic/interface.html#atomic.interface.interface_memory_order">Memory order</a></span></dt>
<dt><span class="section"><a href="atomic/interface.html#atomic.interface.interface_atomic_flag">Atomic flags</a></span></dt>
<dt><span class="section"><a href="atomic/interface.html#atomic.interface.interface_atomic_object">Atomic objects</a></span></dt>
<dt><span class="section"><a href="atomic/interface.html#atomic.interface.interface_atomic_ref">Atomic references</a></span></dt>
<dt><span class="section"><a href="atomic/interface.html#atomic.interface.interface_wait_notify_ops">Waiting and
      notifying operations</a></span></dt>
<dt><span class="section"><a href="atomic/interface.html#atomic.interface.interface_ipc">Atomic types for inter-process
      communication</a></span></dt>
<dt><span class="section"><a href="atomic/interface.html#atomic.interface.interface_fences">Fences</a></span></dt>
<dt><span class="section"><a href="atomic/interface.html#atomic.interface.feature_macros">Feature testing macros</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="atomic/usage_examples.html">Usage examples</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="atomic/usage_examples.html#boost_atomic.usage_examples.example_reference_counters">Reference
      counting</a></span></dt>
<dt><span class="section"><a href="atomic/usage_examples.html#boost_atomic.usage_examples.example_spinlock">Spinlock</a></span></dt>
<dt><span class="section"><a href="atomic/usage_examples.html#boost_atomic.usage_examples.singleton">Singleton with
      double-checked locking pattern</a></span></dt>
<dt><span class="section"><a href="atomic/usage_examples.html#boost_atomic.usage_examples.example_ringbuffer">Wait-free
      ring buffer</a></span></dt>
<dt><span class="section"><a href="atomic/usage_examples.html#boost_atomic.usage_examples.mp_queue">Lock-free multi-producer
      queue</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="atomic/limitations.html">Limitations</a></span></dt>
<dt><span class="section"><a href="atomic/porting.html">Porting</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="atomic/porting.html#atomic.porting.unit_tests">Unit tests</a></span></dt>
<dt><span class="section"><a href="atomic/porting.html#atomic.porting.tested_compilers">Tested compilers</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="atomic/changelog.html">Changelog</a></span></dt>
<dt><span class="section"><a href="atomic/acknowledgements.html">Acknowledgements</a></span></dt>
</dl>
</div>
<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="atomic.introduction"></a><a class="link" href="atomic.html#atomic.introduction" title="Introduction">Introduction</a>
</h2></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="atomic.html#atomic.introduction.introduction_presenting">Presenting
      Boost.Atomic</a></span></dt>
<dt><span class="section"><a href="atomic.html#atomic.introduction.introduction_purpose">Purpose</a></span></dt>
</dl></div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="atomic.introduction.introduction_presenting"></a><a class="link" href="atomic.html#atomic.introduction.introduction_presenting" title="Presenting Boost.Atomic">Presenting
      Boost.Atomic</a>
</h3></div></div></div>
<p>
        <span class="bold"><strong>Boost.Atomic</strong></span> is a library that provides
        <code class="literal">atomic</code> data types and operations on these data types,
        as well as memory ordering constraints required for coordinating multiple
        threads through atomic variables. It implements the interface as defined
        by the C++11 standard, but makes this feature available for platforms lacking
        system/compiler support for this particular C++11 feature.
      </p>
<p>
        Users of this library should already be familiar with concurrency in general,
        as well as elementary concepts such as "mutual exclusion".
      </p>
<p>
        The implementation makes use of processor-specific instructions where possible
        (via inline assembler, platform libraries or compiler intrinsics), and falls
        back to "emulating" atomic operations through locking.
      </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="atomic.introduction.introduction_purpose"></a><a class="link" href="atomic.html#atomic.introduction.introduction_purpose" title="Purpose">Purpose</a>
</h3></div></div></div>
<p>
        Operations on "ordinary" variables are not guaranteed to be atomic.
        This means that with <code class="literal">int n=0</code> initially, two threads concurrently
        executing
      </p>
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">function</span><span class="special">()</span>
<span class="special">{</span>
  <span class="identifier">n</span> <span class="special">++;</span>
<span class="special">}</span>
</pre>
<p>
        might result in <code class="literal">n==1</code> instead of 2: Each thread will read
        the old value into a processor register, increment it and write the result
        back. Both threads may therefore write <code class="literal">1</code>, unaware that
        the other thread is doing likewise.
      </p>
<p>
        Declaring <code class="literal">atomic&lt;int&gt; n=0</code> instead, the same operation
        on this variable will always result in <code class="literal">n==2</code> as each operation
        on this variable is <span class="emphasis"><em>atomic</em></span>: This means that each operation
        behaves as if it were strictly sequentialized with respect to the other.
      </p>
<p>
        Atomic variables are useful for two purposes:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
            as a means for coordinating multiple threads via custom coordination
            protocols
          </li>
<li class="listitem">
            as faster alternatives to "locked" access to simple variables
          </li>
</ul></div>
<p>
        Take a look at the <a class="link" href="atomic/usage_examples.html" title="Usage examples">examples</a>
        section for common patterns.
      </p>
</div>
</div>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"><p><small>Last revised: December 02, 2021 at 06:55:19 GMT</small></p></td>
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